1. Field of the Invention
The present invention relates to a solar power generation system having a cooling mechanism. More particularly, the present invention relates to a solar power generation system in which a solar cell is installed and which is provided with a cooling mechanism capable of cooling said solar cell depending on an output of said solar cell.
2. Related Background Art
In recent years, as an energy source which is safe and does not bring about a load to the environment, a solar power generation system in which a solar cell is used has been spotlighted. However, such a solar power generation system is necessary to be more advantageous in comparison with conventional power generation systems also in terms of the economical viewpoint in order that it can be more widely used. Because of this, various studies have been conducted in order to develop a solar cell having an improved photoelectric conversion efficiency and capable of being produced at a reasonable production cost and which makes it possible to establish a solar power generation system having a high power generation efficiency at a reasonable cost.
Incidentally, in order for a solar cell to have a large output energy, it is important that the solar cell is made to have a large photoelectric conversion efficiency. Besides, it is important to contrive such that the generated energy of the solar cell is increased. In order to increase the generated energy of the solar cell, there is considered, for instance, a measure that the solar cell is maintained at a temperature which is as low as possible. Specifically, in the case where the solar cell is installed outdoors, when the solar cell receives direct sunlight, the temperature thereof is risen, where there is a phenomenon in that the effective power generation efficiency of the solar cell is reduced due to the temperature rise in comparison with that when the solar cell is maintained in a rated state (where the solar cell is maintained at 25° C.). In order to prevent occurrence of this phenomenon, it is necessitated that the solar cell is maintained at a temperature which is as low as possible. In the case where the solar cell is exposed to direct sunlight in summer time, the temperature of the solar cell generally reaches 80° C. or more, where when the solar cell is a silicon series solar cell (such as a crystalline silicon series solar cell or an amorphous silicon series solar cell), the temperature coefficient of the photoelectric conversion efficiency thereof is about −0.4%/° C. (which is meant that the absolute value of the photoelectric conversion efficiency is reduced by about 0.4% per a temperature rise of 1° C.) and because of this, the power generation efficiency thereof is reduced by more than 20%. Therefore, even when a silicon series solar cell having a sufficiently high photoelectric conversion efficiency should be used, unless the silicon series solar cell is adequately cooled, it is difficult for the solar cell to achieve a satisfactory power generation efficiency. Further, in the case where the solar cell is maintained at a relatively high temperature, heat load to the components thereof is increased and accordingly, the durability of the solar cell is deteriorated. Also in view of preventing the durability of the solar cell from being deteriorated, particularly in the case where the solar cell is installed outdoors, it is necessary to cool the solar cell so that the solar cell can be maintained at a temperature which is as low as possible.
Now, in recent years, from the viewpoint of attaining a solar power generation system having a high power generation efficiency at a reasonable cost, a solar power generation system having an optical concentration type solar cell provided therein has started receiving the public attention. In the case where such an optical concentration type solar cell is used, there are advantages such that the number of solar cells, which are the most expensive of the components constituting the solar power generation system, can be diminished. This situation makes it possible to attain a solar power generation system having a high power generation efficiency at a reasonable cost.
In a solar power generation system, even when a relatively small number of optical concentration type solar cells are used, light with a large intensity is impinged into the solar cells to generate a large voltage, where the proportion of the output power energy to the incident light energy, that is, the photoelectric conversion efficiency is improved. Thus, there can be achieved a relatively large power output. Specifically, for instance, when a case wherein a prescribed number of optical concentration type solar cells are arranged on a prescribed area is compared with a case wherein a prescribed number (which is the same as the former number) of solar cells which are not of the optical concentration type are arranged on a prescribed area (which is the same as the former area), the power outputted in the former case is significantly greater than that in the latter case. Even in the former case, in order to achieve a sufficient power output by sufficiently increasing the photoelectric conversion efficiency, it is necessary that an optical focusing system with a high magnification is adopted and a sun-tracking mechanism is provided. However, in this case, the temperatures of the solar cells are more increased in comparison with those when the optical focusing of sunlight is not performed and therefore, it is necessitated to more efficiently cool the solar cells.
In view of the above situation, there has been made a proposal of performing forcible (intentional) cooling for the solar cells in a solar power generation system in order to lower the temperatures of the solar cells. For instance, Japanese Unexamined Patent Publication No. Hei.9(1997)-213980 (hereinafter referred to as JP '980) discloses a forcible cooling means for intentionally cooling the solar cells in a solar power generation system.
The term “forcible cooling means” is meant a means for intentionally cooling an object to be cooled, which is distinguished from a means for cooling said object by way of spontaneous heat radiation or the like.
Particularly, the forcible cooling means disclosed in JP '980 is of the method of performing continuous cooling, where the same energy is used in the forcible cooling for the solar cells regardless of whether solar irradiation is relatively large or small. However, this method has shortcomings such that when the cooling function is designed to conform the time when the solar radiation becomes maximum, excessive energy is consumed when the solar radiation is low and therefore, there entails a disadvantage in that the energy is wasted; and reversely, when the cooling function is designed to conform the time when the solar radiation is weak, there will entail a problem in that the solar cells cannot be sufficiently cooled when the solar radiation is strong.
In order to solve these shortcomings, Japanese Unexamined Patent Publication No. Hei. 5(1993)-83881, Japanese Unexamined Patent Publication No. Hei. 7 (1995)-36556, and Japanese Unexamined Patent Publication No. Hei. 10(1998)-101268 propose a method wherein the temperature of a solar cell to be cooled is detected by a temperature-detecting means and when the detected temperature exceeds a prescribed value, a forcible cooling means such as a fan or the like is actuated to cool the solar cell. However, such a method has shortcomings such that the temperature-detecting means is specially provided and this makes the system costly; when a failure is occurred at the temperature-detecting means, the failure sometimes will cause a failure at the solar cell; and because the cooling effect of the forcible cooling means is always constant, the foregoing problems relating to excess and deficiency of the cooling extent cannot be sufficiently solved.
As a measure to solve such shortcomings as above described, Japanese Unexamined Patent Publication No. Hei. 7(1995)-240532 (hereinafter referred to as JP '532) proposes a method wherein a cooling fan is electrically serialized with a circuit extending from solar cell which are electrically connected with each other and cooling by means of the cooling fan for the solar cells is performed in proportion to a current value generated by the solar cells. However, in general, the cooling effect is not proportional to the power, voltage, current and the like required for the cooling. Therefore, the method described in JP '532 has shortcomings such that when the cooling system is designed such that the cooling effect to the solar cells becomes optimum when the solar cells are in a maximum power generation state, excess or deficiency of the cooling is occurred at an intermediate stage between the case where the solar cells are in a non-power generation state and the case where the solar cells are in a maximum power generation state, where when the solar cells are excessively cooled, the cooling energy is wasted, and when the solar cells are deficiently cooled, excessive temperature rise is occurred in the atmosphere surrounding the solar cells to impart a detrimental effect to the system.